I have found the answer in the wiki of the project
The text passed is the literal block, unrendered.
But it provides a Mustache_LambdaHelper
that can be used to render the text passed.
So I have to add this to my lambda function:
$data->stars = function($label, Mustache_LambdaHelper $helper) {
$aux = "";
$level = $helper->render($label);
$l = intVal($level);
for ($i = 0; $i < $l; $i++) {
$aux .= "+";
}
for ($i = $l; $i < 5; $i++) {
$aux .= ".";
}
return $aux;
};
And that's all it's needed to make it works. Thanks to all readers!